SharePoint redirect for site collection is possible with IIS virtual site creation. This is not possible with AAM Configuration.If you want users to use http://intranet.domain.com/ and have it forwarded to http://intranet.domain.com/sites/portal, there are several ways you can do it.
One easy way is to use IIS directly and not involve Sharepoint at all.
Create a new virtual server in IIS Manager (New Web Site) and give it a descriptive name like "Sharepoint Redirect".--> choose the default settings when running the wizard and select a local directory (any path is fine). -> after the virtual server is created, edit the properties, open the "Home Directory" tab and click "A redirection to a URL", then type your Sharepoint URL in the "Redirect to:" box.